First Gyro:
http://www.foodnetwork.com/recipes/alton-brown/gyro-meat-with-tzatziki-sauce-recipe/index.html
Second, roasting whole lamb on a spit.
We usually end up with 55-65lbs whole lamb from the meat market depending on how late in the season Greek Easter lands, I can't help with live weight. But, that is more than enough for 40+ people when you have spanikopita, pastichio, dolma, pilaf, salad, bakalava, etc.....
Prepare the night before. You will need a lot of peeled garlic cloves, cut larger ones in half. Prepare a bowl or two of salt, pepper, and oregano for seasoning. We eyeball it, so can't give measurements but you cannot have too much oregano. Stab the meaty parts of the lamb and put some seasoning in the hole, followed by garlic, followed by seasoning. Be careful of the fleshy stomach flap you cut that and it falls apart too soon.
Once seasoned and garlic'd spit the lamb. In the cavity throw in a bunch of quartered lemons (leave peels on) and any leftover garllic and seasoning. Sew up the cavity.
Set aside in garage or on spit wrapped in foil until morning, spit is a good idea because only one person gets up at the god awful hour. Around 5-6am start fire and start cooking. Set up heat under ends and move towards the center as cooking proceeds, the ends take longer to cook than the middle.
Baste with mix of olive oil, lemon juice, salt, pepper and oregano, as often as you fell like. If available tie fresh oregano stalks around a brush for basting.
For that extra special flavor cook over grape stumps.
Greeks cook well done, so should be ready around 1-2pm, it will NOT be dry. Exact cook time depends on how many cooks, friends, and relatives are around to argue about whether it is done or not, and how long it takes to come to a consensus. Stoke the fire in the middle about an hour before completion to crisp up the skin.
Remove from rotisserie and lean spit with carcass against tree with drip pan to cool. At which point the vultures will pick off the skin. This is Ok, but no picking off meat. Once cool enough to carve without burning one self. Move to carving table and serve.